The manufacture of meltblown fibrous webs has been discussed in many references, including, Wente, Van A., Superfine Thermoplastic Fibers, 48 Industrial Eng. and Chem. 1342-46 (1956); Report No. 4364 of the Naval Research Laboratories, published May 25, 1954, entitled Manufacture of Superfine Organic Fibers, by Wente, V. A., Boone, C. D., and Fluharty, E. L.; and U.S. Pat. No. 3,971,373 to Braun.
In making meltblown fibrous webs, a thermoplastic polymer or resin is commonly extruded through a row of small, side-by-side orifices into a high velocity gaseous stream that attenuates the emerging material into fibers. The gaseous stream creates turbulence that randomly entangles the fibers to form a coherent nonwoven web on a collector. The collector may be a moving flat belt or rotating cylindrical screen or drum. The resulting nonwoven web is transferred from the collector to a temporary storage roll.
Known processes have a couple drawbacks, namely, they can produce significant waste as a process by-product and they can produce non-uniformities across the web.
Waste (also referred to as weed) is commonly produced at the web edges when manufacturing meltblown fibrous webs. The waste or weed results because the web edges are typically "feathered", meaning the edges taper off and do not have the same weight and density as the central portion of the web. The feathering stems from fiber dispersal at the web edges. To eliminate this variation in weight and density, the web edges typically are trimmed off and then discarded as waste, while the central portion of the web is retained for further processing. The wasted material adds to processing costs, especially when in-line web processing is desired.
Known meltblown fibrous webs are typically mono-layer webs that, by definition, have only a single layer. Mono-layer meltblown fibrous webs often suffer from non-uniformities over their cross-web dimension due, for example, to variations in orifice diameter. The variations in orifice diameter can cause non-uniform fiber deposition that, in turn, causes variations in the basis weight in the cross-web dimension. The basis weight is the weight per unit area of the mono-layer web, and it is commonly adjusted by varying the polymer extrusion rate or the collector speed or both. For example, if a higher basis weight web is desired, the collector speed can be reduced and/or the extrusion rate can be increased. Conversely, if a lower basis weight web is desired, the collector speed can be increased and/or the extrusion rate can be decreased.
One approach to overcoming variations in basis weight include laminating multiple webs together using agents such as adhesives or resins and/or by physical processing such as welding. The variations in the multiple webs then preferably average out the non-uniformities such that the minimum acceptable basis weight is achieved over the entire laminated web. One disadvantage to this approach is that some areas of the web can have an excessive basis weight and hence unnecessary amounts of web material. The unnecessary material, as well as the laminating agents and/or processing needed to laminate the webs to form the multi-web products adds to production costs and increases complexity. Furthermore, the agents and/or welds used to laminate the layers can adversely affect the resulting articles' conformability and flexibility.
Attempts to employ tubular fibrous web processes to achieve a flat web have typically involved forming the tubular meltblown fibrous web and compressing the tube to obtain a flat web without feathered edges. Alternatively, the tubular web may be slit longitudinally so that the tube is opened, thereby producing a flat web with two machine-cut edges. Two such approaches are described in U.S. Pat. No. 3,909,174 (Blair et al.) and U.S. Pat. No. 4,032,688 (Pall). A disadvantage of these processes is that variations in web thickness may often be helical in nature. As a result, slitting the web longitudinally often causes banded variations in the web density, which variations are located at an angle, commonly referred to as a "bias angle," with respect to the web centerline.